Septic shock is the most serious complication of sepsis, a disorder that occurs when the body responds to an infection. Shock, including septic shock, is characterized by blood flow that is inadequate to meet tissue oxygen demand. Prompt recognition of inadequate organ and tissue blood flow, known as hypotension and hypoperfusion, is essential for timely treatment and improved outcome in shock related disorders. Thus, tissue oxygenation may be monitored as a means of monitoring and diagnosing shock, sepsis and other types of infections, as well as monitoring a patient's overall health.
Previously, there were two basic kinds of oxygenation measurements—hemoglobin oxygen saturation in the blood and transcutaneous partial pressure of oxygen. Hemoglobin oxygen saturation in the blood (SO2, SaO2, SpO2), expressed as a percent, is the oxygen present on the hemoglobin in circulating blood divided by the total possible oxygen that could be carried by the hemoglobin. Transcutaneous partial pressure of oxygen (PO2) measures the amount of oxygen drawn to the skin's surface by a heated sensor and provides an estimate of arterial partial pressure of oxygen.
StO2 is the quantification of the ratio of oxygenated hemoglobin to total hemoglobin in the microcirculation of skeletal muscle, and is an absolute number. The measurement of StO2 is taken with a noninvasive, fiber optic light that illuminates tissues below the level of the skin. One way to illuminate tissue below the level of the skin is known as near infrared spectroscopy (NIRS), which uses specific, calibrated wavelengths of near infrared light to noninvasively illuminate the tissue below the skin. These wavelengths of light scatter in the tissue and are absorbed differently dependent on the amount of oxygen attached to hemoglobin in the arterioles, venules, and capillaries. Light that is not absorbed is returned as an optical signal and analyzed to produce a ratio of oxygenated hemoglobin to total hemoglobin, expressed as % StO2.
In practice, near infrared light penetrates tissues such as skin, bone, muscle and soft tissue where it is absorbed by chromophores (hemoglobin and myoglobin) that have absorption wavelengths in the near infrared region (approximately 700-1000 nm). These chromophores vary in their absorbance of NIRS light, depending on changes in the oxygenation state of the tissue. Complex algorithms differentiate the absorbance contribution of the individual chromophores.
While StO2 correlates well with other accepted means of measuring oxygen saturation, StO2 measurement differs from the SpO2 near infrared measurement provided by pulse oximetry. Pulse oximetry measures the systemic oxygen saturation of arterial blood, and requires a pulsatile flow. In contrast, StO2 measures the oxygen saturation of local muscle tissue and does not require a pulsatile flow.
Furthermore, pulse oximetry measures hemoglobin oxygen saturation prior to delivery to the microcirculation where oxygen is exchanged with the cells. SpO2 is therefore a systemic measure and is fairly constant regardless of whether the measurement site is the earlobe, finger, or big toe. Thus, while measurements of StO2 will change as the conditions of supply and consumption change at the measurement site, measurements of SpO2 will not.
Finally, while near infrared spectroscopy can be used to measure oxygenation at various depths of tissue—skin, subcutaneous tissue, and muscle, transcutaneous PO2 measures the partial pressure of oxygen in the skin only.
It is known that noninvasive hemodynamic monitoring may be able to predict outcome in trauma, including shock and sepsis. While methods of making such predictions in relation to pulse oximetry and transcutaneous PO2 are widely known, such methods are not available in relation to StO2. Furthermore, transforming a measurement of tissue oxygenation into a convenient and usable format relating to a patient's oxygenation capabilities is time consuming and tedious. Complex algorithms may be necessary to transform the data, and comparison of both raw data and transformed data to normal or standardized values for evaluating the patient's condition may require charts or other tools.
What is needed, then, is an improved method of using tissue oxygenation data, including StO2 data, to quickly and easily inform a physician about a patient's tissue oxygenation capabilities. This information may then be used by the physician to diagnose and monitor conditions of shock, sepsis and infection as well as to predict outcome in patients suffering from such conditions.